If you simply want the graph to look smoother, add the parameter operator .. as the last parameter to the graph command. To set the sample size of a particular graph to--say--50, add the parameter n=50 to a particular graph command.
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Charles StaatsMar 8 '13 at 13:00

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@microarm15 Please, add the code to your question, rather than in a comment.
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egregMar 8 '13 at 13:13

And in the interest of teaching a man to fish and all that: here's the online documentation. By specifying the function T you can even make the sampling not evenly spaced, if you know a priori where you need higher "resolution".
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Willie WongMar 8 '13 at 13:45

This plot needs quite a large number of samples to be represented correctly. I know you're asking for a way to do this in Asymptote, but I'd like to show a way to generate the plot directly within LaTeX using the PGFPlots package:

Apart from the Asymptote specific things (sampling resolution), the problem is heavily influenced by the eps viewer. As already discussed at My .eps images appear to lack resolution, but turn out crisp after compiling my document, what's going on?, line smoothing in the viewer is the culprit. I get the same rags in the eps plot as you with anti-aliasing on in ghostview, turning anti-aliasing off improved the output quite a bit. That being said, the pdf built from your and egreg's code looks much better, as it is rendered by poppler. Also, in print even the eps plot might look smooth.